Resonant converters have been used in power conversion equipment for some time and in various forms. Most of the early work was done using SCR's (Silicon Controlled Rectifiers) as the switching element. SCR's were suited to this type of operation largely due to their power handling capability. The main disadvantage of using SCR's as the switching element was the switching speed. The frequency of operation was limited to less than 10 KHz. Since this low frequency was within the audible range, such equipment was usually very noisy. More recently, there has been interest in operating at much higher frequencies extending into the mega-hertz region. This is possible due to the availability of much faster components such as fast switching transistors, MOS-FETs, GTO's and faster SCR's.
U.S. Pat. No. 4,138,715 to Miller describes a resonant converter with a control device that senses when the energy in a second inductor is zero (i.e., current in the second inductor is zero). In any resonant converter this is important because if the switching device is turned off while there is still current flowing through the inductor, then a switching transient will occur which can destroy the switching device or, if snubbed or otherwise limited, will increase switching losses defeating one of the advantages of resonant switching.
Miller describes an arrangement which compares the resonant capacitor voltage to the DC input voltage. An additional comparator is provided which compares the output voltage to a reference voltage for the purpose of regulating the output voltage. Together, these two voltage comparators serve to turn the switching device on only when the resonant capacitor voltage is at or above the DC input voltage and the output voltage is below the reference voltage. In this manner, the switching device can only be turned on when the capacitor voltage is within the shaded region shown in FIG. 1.
Accordingly, Miller turns the resonant circuit "on" and "off" at some relatively fixed voltage level on the resonant capacitor with this voltage level being dependent on the DC input voltage. The frequency of operation may therefore vary over a wide range to compensate for line and load variations.
In most resonant switching converters, the output voltage or current delivered to the load is controlled by varying the repetition rate at which switch S1 is open and closed. In this case the power capable of being delivered to the load is then approximated as a linear function of the repetition frequency as expressed by ##EQU1## wherein: P.sub.O =Output power
C.sub.1 =Resonant capacitor value PA1 V.sub.C2 =Resonant capacitor voltage at time of switching PA1 f=Repetition frequency PA1 (1) complex control schemes are required to insure proper switching of the resonant currents; PA1 (2) wide frequency variations are required to compensate for line and load variations; PA1 (3) usually cannot operate to no load; PA1 (4) cannot operate over extremely wide input or output ranges without wide variations in the operating frequency; and PA1 (5) changes in the values of resonant circuit components can adversely affect operation.
In most resonant switching converters, the capacitor voltage at the time of switching is relatively constant. If the input voltage were changed, causing a corresponding change in V.sub.C1 at the time of switching, then the power capable of being delivered to the load would vary as a function of V.sub.C1.sup.2, assuming the frequency f is held constant. Thus, a minor shift in the input voltage produces a large shift in the power capable of being delivered to the load. However, varying the input voltage is difficult since this is usually a rectified and filtered DC voltage derived from 50 Hz or 60 Hz mains and would require an intermediate conversion stage.
There are many disadvantages of existing resonant converter topologies such as: